Not applicable.
The present invention relates to a method and device in calendering. The method involves the calendering of a paper or paperboard web in such a manner that after the treatment the web has a smooth surface and a bulky structure. The device is arranged to implement the aforementioned method.
Typically, calenders comprise superimposed rolls, between which a nip is formed. A calender can comprise one or more roll pairs forming a nip. The rolls may be unheated or heated, and they can have a hard or soft surface. The web travels via the nips along a winding path, and by means of this treatment, for example, the variations in the thickness of the web are reduced, the surface becomes smoother and the web becomes thinner.
U.S. Pat. No. 5,524,532 discloses a method and apparatus for calendering a surface of a paper or board web. A temperature difference is created between the web surfaces so that the surface to be calendered is cooler than the opposite surface. Moisture within the web is transferred towards the cooler surface while substantially preventing evaporation of moisture from the web so as to create a predetermined moisture profile. The web having a predetermined moisture profile is advanced into the calendering nip so that the relatively cooler and moister surface of the web is pressed against the heated roll of the nip to thereby calender that web surface.
U.S. Pat. No. 5,649,478 discloses a machine calender by means of which a smooth surface and a bulky structure is attained for the paper or paperboard. Before the nip, moisture is sprayed on the transfer rolls of the machine calender, the moisture being shifted to the web in the nip.
U.S. Pat. No. 5,750,259 discusses the finishing of the web by means of a hot calender roll and a smooth-faced belt, which form a soft nip. The web becomes smooth and glossy.
WO 98/44195 discusses the calendering of a web by means of a device which is arranged to form a long nip. On both sides of the web to be calendered there is a mirror-symmetrical arrangement in which an endless belt preferably made of metal travels around a soft-faced and a hard-faced roll. The soft-faced rolls on both sides of the web form a nip via which the web travels. One embodiment of the invention disclosed in the publication (FIG. 4 in the publication) is a device in which hard-faced rolls also form a nip. In the nip formed by the hard-faced rolls it is also possible to use a very strong press, which corresponds to the effect produced by the machine calender. After the nip formed by the hard-faced rolls, the belts travelling on both sides of the web support the web in such a manner that the web does not bend, said bending being a problem when high press pressures are used. The web travels between the belts to a nip formed by the soft-faced rolls, which nip is longer than the nip formed by the hard-faced rolls, and the pressure is smaller therein. In this nip a glazing result similar to the one attained when using a soft calender is obtained on both surfaces of the paper.
There are situations in which the thinning of the web is not desired in connection with the calendering. In some applications, for example in printing papers, bulky paper with a smooth surface is required. Paper which has dense, smooth outer surfaces and is bulky in the area between the surfaces is stiff, which is a good quality in many applications.
By means of the method and device according to the invention it is possible to produce smooth, bulky paper or paperboard. The method is characterized in that the moist paper or paperboard web travelling under a slight external pressing pressure through a long nip formed between two smooth, moisture impermeable counter surfaces from which at least one is heated, and a first surface of the web is in contact with the first counter surface and a second surface of the web is in contact with the second counter surface, the first counter surface having a first temperature of at least 100xc2x0 C. and the second counter surface having a second temperature of at least 160xc2x0 C., in such a way that the second surface of the web is calendered during travelling between the counter surfaces by the contact with the second counter surface and that the inner vapour pressure inside the moist paper or paperboard web is increased due to the heating of the web and the restriction of the evaporation of moisture from the web by means of the counter surfaces, said inner vapour pressure resisting the slight pressing pressure between the smooth counter surfaces thus counteracting reduction of the thickness of the web during calendering. The device according to the invention is characterized in that it is arranged to carry out the above mentioned method.
By means of the method and device according to the invention it is possible to attain an advantageous smoothness of the surface in such a manner that even in the worst case, the bulkiness is only slightly reduced. Among other things, the product attained by means of the method and device has the advantage that it absorbs printing ink evenly, and it has a flexible structure in such a manner that a good contact is attained between the printing cylinder and the paper. By installing the devices successively, it is possible to treat both sides of the web.
Before calendering, the web is produced by means of a known method and device. In the calendering method according to the invention, a moist paper or paperboard web travels between smooth counter surfaces in such a manner that a substantial external loading is not exerted on the web. The smooth counter surfaces form a long nip which is only slightly loaded externally, the loading of the nip being constant on the entire length of the nip. The counter surfaces are made of moisture impermeable material and they restrict the evaporation of moisture from the web. The first counter surface is heated to a given temperature T1 and the second counter surface to a given temperature T2, wherein the temperature T2 is higher than T1. Because the space between the counter surfaces is substantially closed, steam starts to flow towards the counter surface which is in a lower temperature, said temperature being T1. Thus, the steam produced on the hotter surface in connection with drying shifts towards the lower pressure, i.e. towards the counter surface which is in a lower temperature. In order to produce vapour pressure, it is required that the temperature T1 is at least 100xc2x0 C. The surface of the web becomes smooth in the contact against the hotter counter surface. Against the counter surface in the lower temperature a pressure is exerted, said pressure resisting the pressing pressure between the smooth counter surfaces. Advantageously, the inner vapour pressure is allowed to be reduced in the contact against the smooth counter surface, wherein a slow and controlled reduction of the vapour pressure is attained inside the paper. Thus, it is ensured that the surface of the paper remains smooth, and fibres are not discharged from the surface of the web. The web is in a direct contact with both counter surfaces without a wire therebetweeen, which wire would cause markings on the web. The surface roughness of the smooth counter surfaces is advantageously Ra =0.1-0.4 mm. After the calendering treatment the web is dried further, if necessary, and thereafter the web is possibly reeled.
The method according to the invention can also be used for placing additives on the surface of the web. Pigments of suitable particle size and/or fines are conveyed on the surface of the web by means of inner vapour pressure, wherein they fill the spaces between the fibres and/or pores of the fibres. Thus, the paper is in a way coated from inside. The method according to the invention can also be combined to multi-layer forming, wherein the raw material composition of the surface layers can be optimized to attain an even and smooth surface structure. In multi-layer forming, pigments and/or fines can advantageously be placed only on the surface layer.
The high temperature required in the method can be attained for example by means of known thermoroll technique, or by means of a hot drying cylinder. Advantageously, the treatment is conducted by means of a device according to the invention, in which smooth counter surfaces are heated by methods known as such.